Film music has power over its audience. Why? There is a body of theory about the sources of film's powers and pleasures, and the way in which they are tied to the presence of music in film. ‘Why does film music work?’ introduces some important theories of film music and the theorists who espouse them, and presents a brief history of the marriage of film with music. Originally, music compensated for the lack of sound in film and covered up the noise of the projector. Why has music in film survived? It is easier to understand how film music works than to understand why it works. What made it indispensable?